1. Field of the Invention
The invention relates to array microphones, and more particularly to phase mismatch calibration of output signals of array microphones.
2. Description of the Related Art
An array microphone is an apparatus comprising a plurality of microphones. When a sound propagates to a vicinity of the array microphone, each of the microphones of the array microphone respectively converts the sound to a microphone signal, thus obtaining a plurality of microphone signals. Due to slight spatial differences in sound receiving locations of the microphones, the microphone signals have a slight phase difference therebetween. A beamforming module can therefore determine a spatial direction of the sound according to the phase differences between the microphone signals and attenuates noise and interference coming from other directions. Thus, a target signal comprising a more desired sound component and less noise and interference is obtained.
Because the beamforming module determines the spatial direction of the sound according to the phase differences between the microphone signals, accuracy of the phase difference between the microphone signals determines precision of beamforming. The phase differences between the microphone signals generated by the array microphone, however, comprises delays resulting from circuit differences of the microphones rather than spatial differences in sound receiving locations of the microphones. The delays caused by circuit differences among different the microphones degrades precision of beamforming. Thus, a phase calibration module is required to compensate output signals of an array microphone for delays caused by circuit differences of microphones of the array microphone.
A conventional phase calibration module directly determines delays caused by circuit differences of microphones according to output signals of the microphones. The circuit differences of microphones of an array microphone, however, causes a much longer delay in low-frequency components of the microphone output signals than in high-frequency components of the microphone output signals. The low frequency components of the microphone output signals therefore have a greater signal distortion and phase mismatch than the high frequency components of the microphone output signals. Because the conventional phase calibration module does not differentiate between the low frequency components from the high frequency components in delay calculation and compensation, delays due to circuit differences cannot be compensated with a high precision, degrading performance of subsequent beamforming. Thus, a method for phase mismatch calibration for an array microphone is required.